1. Field of the Invention
The present invention relates to a method and to an equipment for the detection of mechanical injuries in the lumbar spine of a patient and for the identification of these injuries using a mathematical model representative of the physiological behavior of the spine of a human being.
2. Brief Description of the State of the Art
It is well known in the medical art that common back disorders have a mechanical etiology. It is also well known from pathological studies that there are two common patterns of disc injury which correspond to two different types of mechanical failure of the spine.
The first type of common injury hereinafter referred to as "compression injury", usually starts by a central damage to the disk with fracture of varying magnitude of the end plates of the adjacent vertebrae, sometimes followed by injection of part of the nucleus into the vertebral body. In this particular case, the injured end plate permits the invasion of the avascular nucleus and of the avascular inner portion of the annulus by granulation tissue ingrowing through the fractured end plate, such an invasion leading to gradual destruction of the avascular nucleus and inner annulus. In the early stages, the facet joints of the vertebrae are not affected and the outer annulus survives while the center portion of the disc is destroyed. With progression, the disc loses its thickness while the outer layer of the annulus remains relatively well preserved. With lost of disk thickness, the facet joint subluxates and develops a moderate degree of osteoarthritis.
Usually, the fracture of the end plate of a vertebra is an undisplaced fracture of cancellous bone which heals rapidly. The symptoms are short lived, typically lasting two weeks. The facet joint arthritis appears late. At this stage, symptoms may also arise from the reduction in size of the spinal canal (lateral or central spinal stenosis).
The other type of common injury hereinafter referred to as "torsional injury", is characterized by a damage to the annulus occuring simultaneously with a damage to the facet joints. The annulus is avulsed from the end plate and its laminae become separated while the central disk and the end plate remain intact. At the later stage, the annulus develops radial fissures while the nucleus remains relatively untouched. The changes in the facet joints are severed with massive joint destruction and osteophytosis similar to hypertropic arthritis. Relatively late in the process, there may be changes in the end-plates and central disks, with consequent collapse of the articular surfaces and chronic synovitis.
In this particular case, the basis injury is to collageneous ligamentaous tissue which requires six weeks to regain 60% of its strength. Because the injury involves both the disk and facet joints, it is more difficult for the joint to stabilize itself and recurrence is frequent. The condition is progressive and may lead to spinal stenosis, instability and degenerative spondilolisthesis.
Tests conducted in laboratory have shown that a compression injury is easily produced by compressing a joint between 2 Mpa to 6 Mpa. A torsional injury can be seen with as little as 2 to 3 degrees of forced rotation requiring only 22 to 33 Newton-meters of torque.
Statistically, in a group of patients suffering from back disorders, 64% exhibit torsional injuries whereas 35% exhibit axial compression injuries. Statistics have also shown that torsional injury occurs mainly at the L.sub.4 -L.sub.5 level (almost 76% of forth joint problems are of torsional nature). Statistics have also shown that almost 98% of the compression injuries occur at the L.sub.5 -S.sub.1 level. Statistics have further shown that double injuries where the joint is injured both in compression and torsion, occur in 22% of the cases, invariably at the L.sub.5 -S.sub.1 level.
The following Table I reflects the probabilities of injuries among patients complaining from backache and sciatica, or sciatica alone. As can be seen from this Table, the important frequency of torsional injury cannot be overlooked. As can also be seen, the probability of a third type of injury giving symptoms is very remote.
TABLE I ______________________________________ CLINICAL DETERMINATION OF THE VARIOUS PROBABILITIES OF INJURIES JOINT P (injury) P (compression) P (torsion) ______________________________________ L.sub.5 /S.sub.1 47% 98% 22% L.sub.4 /L.sub.5 47% 1%&lt; 76% L.sub.3 /L.sub.4 5%&lt; 1%&lt; 1%&lt; L.sub.2 /L.sub.3 1%&lt; 1%&lt; 1%&lt; L.sub.1 /L.sub.2 1%&lt; 1%&lt; 1%&lt; 100% 100% 100% ______________________________________
It is also well known that health professionals are trained to use symptoms in the determination of diagnoses, the large numbers of known symptoms being quite naturally associated with a large number injuries and diagnoses. Unfortunately, as can be understood from the above short description of the pathology in the case of back disorders, both the compression and torsion injuries give rise to identical symptomology. Hence, symptoms cannot be used to diagnose a type of injury because identical symptoms may arise from different injuries.
It is also well known in the art that low back pain is the leading cause of disability in North America today, affecting from 8 to 9 million people. It is the most common disability in persons under the age of 45 and the third only after arthritis and heart disease in those over 45. It is also estimated that two of three persons will have a low back pain at some time of their lifes, usually between the ages of 20 and 50. The fact that problems are so common in people of working age is not coincidental. Indeed, most of the back problems are work-related. As the injury caused by a certain task cannot be identified from the patient's symptoms, it is of course not possible to relate directly a given task to an injury mode, although such a relationship is central to the definition of tasks that will not injure a specific worker.
The economic effects of back pain and injuries are staggering. Back problems are second only to the common cold as a cause of absenteism in the industry. It is moreover responsible for 93 million lost workdays every year and is a leading cause of reduced work capacity. Hence, an incentive for prevention of back injury is very large.
In order to unequivocally relate a given task to a given injury in the absence of any measurement of the effect of the task on a given joint, it has already been suggested to use mathematical and/or biomechanical model of spine, like the one suggested by J. M. Morris et al in their article "The Role of trunk in stability of the spine, J. Bone and Joint Surg., 43A, 1961. However, a major problem with the known models of spine, including the widely used model of J. M. Morris et al, is that they do not truly reflect the physiological behaviour of the spine over the full range of capacity.
Thus, by way of example, the model of J. M. Morris et al which assumes, as fundamental hypothesis, that the moment generated by the body weight and any external load carried by the patient is balanced by the combined action of the erectores spinae and the intra-abdominal pressure, is a very poor representation of physiological behaviour which is not supported by observations. By way of example, such a model predicts a total failure of the spinal mechanism at about one fourth of the known potential of a healthy spine.
The major reason why all of the models known to the inventor are defective is essentially because they give an incomplete representation of the actual anatomy of a human being. It is true that a moment-supporting member is required in such a model but this cannot be the abdominal pressure only, as suggested by J. M. Morris et al.